1. Field of the Invention
The present invention relates to a battery voltage monitoring apparatus for detecting a battery voltage of a power supply device having a plurality of secondary batteries connected in series, and particularly to a battery voltage monitoring apparatus for detecting a disconnection of a signal line for measuring a voltage.
2. Description of the Related Art
Conventionally an electric and hybrid vehicles are known as an environmentally friendly vehicle. A motor is used for a drive source of the electric and hybrid vehicles. A rechargeable secondary battery is connected with the motor. A direct current obtained from the secondary battery is converted into an alternating current by an inverter to drive the motor. Since the secondary battery requires a high voltage, it is usually formed as a battery pack having a plurality of battery cells connected in series.
A plurality of voltage sensors are used to detect voltages of each battery cell in the battery pack. The voltage sensors are divided into an appropriate number to be modularized. A plurality of the voltage sensor modules are prepared and connected in series in case many battery cells to be connected in series are required for the electric vehicles, for example. An apparatus for monitoring a voltage of such a battery pack is disclosed in Japanese Unexamined Patent Application Publication No. 2003-208927, Japanese Unexamined Patent Application Publication No. 2003-111284 and Japanese Unexamined Patent Application Publication No. 2005-117780. An apparatus for monitoring a voltage of the battery pack including a plurality of integrated circuits (ICs) are explained hereinafter in detail. The modules having the plurality of voltage sensors are formed by one integrated circuit.
FIG. 5 is a schematic view showing a conventional voltage monitoring apparatus. As shown in FIG. 5, one IC is capable of detecting voltages of four battery cells. Each input terminal of the IC is connected to battery cells C11 to C18 through lines for measuring voltage L11 to L19. The IC11 of FIG. 5 operates with a positive electrode side of the battery cell C11 (node N11) as a power supply potential, and a negative electrode side of the battery cell C14 (a positive electrode side of the battery cell C15, node N12) as a ground potential. The IC detects an overvoltage or undervoltage condition of the battery cell it is monitoring and outputs an overvoltage or undervoltage detection signal. A disconnection of a highest line supplying power to an IC such as the line L11 in an apparatus including the plurality of ICs connected in series for monitoring the voltage of the battery pack could cause the following problem.
A disconnection of the line L11 of FIG. 5 stops a power supply to a node N13, thus a voltage of the node N13 decreases. As a result, parasitic transistors in the integrated circuit are turned on, causing a latch up.
It has now been discovered that the integrated circuit forming the voltage monitoring apparatus could be broken down by heat caused by the latch up as a result of the disconnection of the line.